The esters of 2-ethylhexanol, especially the phthalate, are among the most commonly used plasticizers. The alcohol is obtainable by, for example, subjecting propene to hydroformylation, dimerizing the resulting butanal by the aldol reaction, a term which is used throughout this specification, including the claims, as including the subsequent dehydration to an unsaturated aldehyde, and hydrogenating the resulting aldehyde to form a saturated alcohol.
The propene, produced for example by a steam cracking plant, has to be purified before hydroformylation, and its cost as feedstock is increased as a result.
Although the plasticizer esters derived from 2-ethylhexanol are widely used, for some purposes, for example where a lower volatility, or a stronger solvator for the polymer is needed, higher molecular weight esters, for example those based on nonanol, are preferred. The C.sub.9 esters presently available commercially are typically derived from an isomeric mixture of C.sub.9 alcohols and the users' requirements for product consistency may result in manufacturing complexities.
These complexities result from variations in feed composition and reaction conditions in the process for the manufacture of the precursors to the alcohols. These precursors may be formed for example by oligomerizing a mixed C.sub.3 to C.sub.5 olefin feed, giving a mixture of linear and branched olefins, predominantly having six to ten carbon atoms, from which is distilled a mixed C.sub.8 olefin, which is in turn hydroformylated (oxonated) and hydrogenated to form the isomeric C.sub.9 alcohol mixture.
In other commercial processes, the C.sub.9 alcohol precursors are typically obtained by dimerizing butene streams and oxonating the resulting C.sub.8 olefin fraction. The butene stream itself contains a mixture of isomers, in proportions that may vary over a period, and the cobalt oxo process causes some isomerization. Thus the alcohols resulting from hydrogenation of the aldehyde form a reaction product of variable isomer distribution together with lower and higher homologues, necessitating further treatment if customers' product specifications are to be met.
In a typical commercial process for the manufacture of a plasticizer ester, the alcohol is employed in excess over the acid, and alcohol is stripped from the ester product and recycled. Any impurities and any less reactive isomers tend to concentrate in the reaction vessel as the reaction progresses, resulting in a change in the composition over time. In turn, the downstream users' quality control inspection of the incoming product is more onerous than if it were a single isomer.
Processing of thermoplastics containing a multi-isomer plasticizer may be more difficult to control in certain applications, resulting in a greater possibility of inconsistencies in properties between different batches of the final product.
This in turn may require the user to have tighter control over process variables, e.g., oven temperature ranges in motor vehicle paintshops and flooring material lines, than would otherwise be necessary, and also complicates material recycling.
In applications employing the corresponding acids, there is an even greater requirement for purity, for example when the acids are being employed in synthetic lubricant manufacture, or in peroxide polymerization initiator manufacture.
Finally, effluent and environmental monitoring is more difficult; e.g., a single isomer material may have a minimum detectability an order of magnitude lower than a multi-isomer material.
There accordingly remains a need for an alternative route to commercially useful organic molecules, and more especially one that provides flexibility and a greater control of product structure, particularly the ability to produce single isomers if desired.
In addition there remains a need for a route sufficiently flexible to be able to use different feedstocks of varying purity, particularly feedstocks from the various natural gas sources emerging around the world.